program p1;
type tip=0..1000000000;
var a,b:array[1..100] of integer;
    f1,f2:text;
    n,j,i,l,m:longint;
    k,aux,max:tip;
    ok:boolean;
    begin
    assign(f1,'carnati.in');
    reset(f1);
    readln(f1,n,k);
    for i:=1 to n do
    readln(f1,a[i],b[i]);
    close(f1);
    repeat
    ok:=true;
    for i:=1 to n-1 do
    begin
    if a[i]>a[i+1] then
    begin
    ok:=false;
    aux:=a[i];
    a[i]:=a[i+1];
    a[i+1]:=aux;
    aux:=b[i];
    b[i]:=b[i+1];
    b[i+1]:=aux;
    end;
    end;
    until ok=true;
    for i:=1 to n do
    for j:=1 to n do
    for l:=j to n do
    begin
    aux:=0;
    for m:=j to l do
    if b[i]<=b[m] then
    aux:=aux+b[i];
    aux:=aux-k*(a[l]-a[j]+1);
    if aux>max then
    max:=aux;
    end;
    assign(f2,'carnati.out');
    rewrite(f2);
    writeln(f2,max);
    close(f2);
    end.